[Boatanchors] A story of two grounds.

Philip Atchley [email protected]
Mon, 21 Oct 2002 21:16:01 -0000


Hello all.

Sometime back I re-capped and serviced this '67EAC R-390A that I'm presently
using and it performs VERY well.  I chose to go the "under chassis" route
for the 'lytics rather than rebuild the cans.

One thing that I noticed was that this set had some 120Hz noise in the Local
audio that, while not loud was noticable with the volume turned all the way
down.  NOT a hum, it was more like the line related switching noise one
hears from Triac controlled lamps etc.  So today I decided to troubleshoot
it (I HAVE to get a helper to move this thing around 8^).  I suspected a
ground loop.

To make a long story short.  When I mounted the capacitors under the chassis
I mounted two solder lugs under existing screws, cleaning under them to make
sure they made good contact.  This so I could make a VERY neat installation
of the capacitors as the lugs provided a convenient mounting point for the
ground end of the 'lytics.  I've done this before on other R-390A's with
good luck.  Previous sets had the Solid state rectifiers while this one has
intact tube rectifiers.  Anyway, on a hunch I replaced the ground lugs with
two insulated terminal strips (Radio Shack) in which I had cut off all but
one insulated lug.  I then connected the negative end of the capacitors to
these lugs and ran a ground wire back to the two appropriate Capacitor Octal
sockets giving the capacitors their original ground points.

Upon testing, Voila!  That did the trick.  IF I put my ear on the speaker I
can 'just discern' the noise.  In normal operation certainly not noticable
like it was.

MORAL OF THE STORY:  I'm sure Art Collins spent a lot of money researching
and designing the grounds in our beloved R-390A's, don't try to second guess
him 8^)

One additional Note:  The ground lug where these caps are originally
grounded IS NOT all that far from where one of my ground lugs was located.

73 de Phil  KO6BB

Chasing NDB's (Non-Directional Beacons)with .....
LF Modified R-390A,'67 EAC (26 valves).
Homebrew 'Hybrid' Longwave CW receiver.
20 Meter Long Alpha Delta Sloper Antenna.
50dB Gain Low Noise Tunable Preamp.
Heathkit  HD-11 Q-Multiplier.
Merced, Central California.
37.18N  120.29W  CM97sh.